Implantable devices with reduced needle puncture site leakage

ABSTRACT

A prosthetic implantable device that offers a reduction in fluid loss when the device is punctured, such as by a dialysis needle or suture needle, and the needle is subsequently removed. The device may be made to be thin and flexible, and with longitudinal stretch, in order that it also offers good handling and kink resistance to a surgeon. While the device is preferably of tubular form, flat sheets or other forms may also be made. The device includes inner and outer layers of a porous material having a microstructure of nodes interconnected by bent fibrils, and having void spaces between adjacent bent fibrils. The inner and outer layers are joined by an elastomeric adhesive that may interpenetrate the void spaces of the adjacent surfaces of the inner and outer layers, that is, the inner surface of the outer layer and the outer surface of the inner layer. Optionally, a middle layer of an elastomeric material may also be provided, joined to the inner and outer porous layers by the interpenetrating elastomeric adhesive. The device is preferably a vascular graft and more preferably a vascular graft for kidney dialysis.

FIELD OF THE INVENTION

The present invention relates to the field of implantable devices suchas vascular grafts, patches and the like.

BACKGROUND OF THE INVENTION

A common problem with vascular grafts is bleeding through holespunctured through the wall of a graft by suture needles or dialysisneedles. Commercially available vascular grafts are most conventionallymade of polyethylene terephthalate fabric or porouspolytetrafluoroethylene tubing but materials of biologic origin such ashuman or bovine arteries or veins have also been used. Suture needlesused to create an anastomosis with these vascular grafts typicallyresult in significant bleeding through the resulting holes that must bestopped prior to closure of the operative incision. Dialysis treatmentof individuals suffering from renal failure requires that the blood ofthe individual be withdrawn, cycled through a dialysis machine andreturned to the individual. A common approach to providing the necessaryhemodialysis access is the use of an implanted arteriovenous vasculargraft that may be subcutaneously cannulated by a dialysis needleconnected to a dialysis machine via lengths of tubing. These dialysisneedles may also produce undesirable bleeding at the puncture site upontheir removal.

Vascular grafts presently used for hemodialysis access are typicallyimplanted for about 14 days prior to cannulation with a dialysis needleso that the graft has had time to become surrounded by fibrotic tissueand thereby reduce the risk of hemorrhage about the outer surface of thegraft following removal of the dialysis needle. A vascular graft fordialysis applications that allows early cannulation followingimplantation without compromising other positive characteristics wouldbe a significant step forward in the field of hemodialysis access.

Suture line bleeding resulting from graft penetration by a suture needleis frequently aggravated by tension applied to the sutures duringconstruction of the anastomosis, the tension generally resulting inelongation and enlargement of the hole created by the penetration of thesuture needle. Bleeding through suture holes must be stemmed before theaccess incision can be closed. Suture hole bleeding is thus responsiblefor both increased blood loss and increased time of operation. Avascular graft offering reduced suture bleeding would be of value inboth regards.

An arteriovenous access vascular graft is described by U.S. Pat. No.4,619,641 to Schanzer, which teaches the construction of an access graftcomprising two commercially available expanded polytetrafluoroethylene(ePTFE) tubular vascular grafts in coaxial relationship with a space ofabout 1 mm disposed between the inner and outer grafts. The space isfilled with an elastomer such as silicone. While this construction mayoffer reduced bleeding after withdrawal of a dialysis needle, the graftis stiff and consequently difficult to work with during implantation. Asimilar construction is described by U.S. Pat. No. 6,719,783 to Lentz etal., expressly teaching that the inner and outer ePTFE grafts are ofdissimilar porosity.

Della Corna et al., in U.S. Pat. No. 4,955,899, teach the manufacture ofan ePTFE tubular graft having a coating of an elastomer. The graft ismade by longitudinally compressing an ePTFE tube on a mandrel, andcoating the compressed tube with the elastomer. After removal from themandrel, the resulting graft has some degree of longitudinal compliance.However, providing an exposed outer surface of elastomer is generallydeemed undesirable.

House et al., in U.S. Pat. No. 4,877,661, teach an ePTFE graft thatoffers longitudinal compliance without requiring an elastomer. Thisgraft is made by placing an ePTFE tube on a mandrel and compressing itlongitudinally, and subsequently exposing it to heat. The resultingePTFE tube has bent fibrils (from the longitudinal compression andheat-setting) that act as hairpin springs, allowing for good bendingproperties with kink resistance and longitudinal compliance. While thisgraft is effective as a dialysis graft that bleeds less than aconventional ePTFE graft following the removal of a needle, even lessbleeding would be desirable.

Sowinski et al., US 2004/0024442, teach an ePTFE tubular graft whereinan ePTFE tube is coated with an interpenetrating elastomer andcompressed longitudinally. It is further taught that the coating andcompression steps are interchangeable. A similar process and tube istaught by Tu et al., EP 0256748. U.S. Pat. No. 5,529,820 to Nomi et al.teaches an ePTFE tube provided with an interpenetrating coating of anelastomer on one surface, for use as an endoscope tube.

US Pat. No. 5,061,276 to Tu et al. describes a vascular graft comprisinga composite tube of ePTFE and an elastomer, having an outer layer ofelastomeric polymer fibers wound under tension about the circumferenceof the graft to cause retraction of the tubing from its original size.The wrapping of elastomeric fibers is provided with the intention ofmaking the graft more compliant.

Myers et al., U.S. Pat. 5,628,782, teach an ePTFE vascular graft fordialysis that provides a layer of fibers about the outer surface of anePTFE tubular graft. The fibers are preferably provided with an outercovering of ePTFE film to retain the fibers to the graft surface. Thepresence of the fibers provides a large surface area to any bloodescaping a puncture site, encouraging hemostasis. The fibers result in asomewhat bulky graft with poorer graft handling properties than manyconventional vascular grafts. Another ePTFE vascular graft for dialysisis taught by Silverman et al. in U.S. Pat. No. 5,931,865. A multiplelayer tubular construction is described, wherein one layer is underlongitudinal compression relative to another layer.

Not withstanding the advantages of the above described devices, thereremains a need for vascular grafts and other implantable devices thatoffer improved handling properties to the surgeon and further reducedleakage of body fluids such as blood following puncture by a sutureneedle or a dialysis needle.

SUMMARY OF THE INVENTION

The present invention relates to implantable devices such as prostheticvascular grafts that, compared to conventional commercially availabledevices, offers a reduction in blood loss or other fluid loss when thedevice is punctured by a needle and the needle is subsequently removed.The device may be made to be relatively thin and flexible, and withlongitudinal compliance (stretch), in order that it also offers goodhandling and kink resistance. While the device is preferably of tubularform, flat sheets or other forms may also be made.

The device has particular utility as a vascular graft, and moreparticularly as a vascular graft for kidney dialysis. It may also beuseful for various other implantable device applications such as biliaryor tracheal where a device that is resistant to fluid leakage followingpuncturing with a needle may be desired, such as to limit holes that maybe formed in mounting a stent and graft together using a suture.

The device comprises at least inner and outer layers of a porousmaterial having a microstructure of nodes interconnected by bentfibrils, and having void spaces between adjacent bent fibrils. The innerand outer layers are joined by an elastomeric adhesive thatinterpenetrates the void spaces of the adjacent surfaces of the innerand outer layers, that is, the inner surface of the outer layer and theouter surface of the inner layer. Optionally, a middle layer of anelastomeric material may also be provided, preferably joined to theinner and outer porous layers by the interpenetrating elastomericadhesive. It has been found that good adhesion is obtained betweenlayers with only a small depth of interpenetration into the wall of theporous material.

Preferably, the inner and outer layers of porous material having amicrostructure of nodes interconnected by bent fibrils is expandedpolytetrafluoroethylene (ePTFE). This material has a long history of usein various implant applications, including blood contact applicationssuch as sutures, vascular grafts and stent-grafts. It is believed thatother biocompatible materials with node-and-fibril microstructures mayalso be used, such as polypropylene or ultra-high molecular weightpolyethylene. The elastomeric material may be a material such assilicone rubbers, polyurethanes or fluoroelastomers (such as, forexample, taught by published patent application WO 2004/012783). It isnot required that the elastomeric material be a cross-linked material.The elastomeric material may optionally be porous. For the presentinvention, “elastomeric materials” are considered to be polymericmaterials capable of being stretched in one direction at least tenpercent by the application of a relatively low force and, upon releaseof the force, will rapidly return to approximately the originaldimension (i.e., the dimension that the material had prior to theapplication of the stretching force).

Further, as an alternative to, or in addition to, the elastomericmaterial as a middle layer, the graft may incorporate an additionalcomponent (e.g., metal or plastic) in the construction in a way thatprovides stretch and reduced needle puncture site leakage behavior tothe construction. This can be accomplished by, for example, asuperelastic metal such as a nitinol wire formed into a stent, such astubular braided structure or a metallic wire formed into a ring orhelical structure. This offers good flexibility for bending, transversecompression resistance and provides longitudinal “stretch” into theconstruction.

While the preferred embodiment of the graft is made with two layers ofgraft material and an intermediate elastomeric layer, the graft may alsohave additional layers. For example, three layers of graft material maybe used alternating with two layers of elastomeric material. For allembodiments, the graft layers may all be of the same material or thegraft layers may be different in one or more characteristics (e.g., wallthickness or mean fibril length). Likewise, if more than a singleelastomeric layer is used, the layers may be the same or may havedifferent characteristics. It is also apparent that a graft may be madewith different constructions along different portions of its length.

The graft of the present invention has longitudinal stretch. The lengthof the graft may be extended by applying a slight amount of tension tothe graft (e.g., by hand). After the tension is released, the graft willquickly recover to about the original length prior to the application oftension.

Tubular grafts may have a constant diameter between graft ends, oralternatively may be tapered, whereby one end of a tubular graft has asmaller diameter than the opposing end. Other configurations may also beappropriate, including bifurcated devices and stepped wallconfigurations.

The combination of inner and outer porous tubes with bent fibrils andthe middle layer of elastomeric material offers the highly desirable andheretofore unachieved combination of reduced leakage following removalof a needle from a puncture site, along with good biocompatibility,longitudinal stretch and good handling and bending properties. The goodbending properties appear to be the result of the use of inner and outerporous tubes having bent fibrils, on either side of the elastomericmaterial. The bent fibrils adjacent the outer meridian of a bent tubeare able to extend (unbend or straighten) while the fibrils adjacent theinner meridian of the bent tube are able to bend still further, therebyenabling the tube to bend smoothly without kinking. It also offers goodsuture retention, along with reduced suture line bleeding. Because ofthe reduced bleeding from needle penetration, a vascular graft made inaccordance with the present invention, when used for dialysisapplications, may allow early cannulation during the period of timefollowing implantation that is normally reserved for healing prior toinitial cannulation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view of a typical vascular graft after implantation in ahuman forearm for use as a dialysis graft.

FIG. 2A describes a schematic representation of a node and fibrilmicrostructure of the prior art wherein the fibrils are relativelystraight.

FIG. 2B is a scanning electron photomicrograph (500× magnification) of anode and fibril microstructure of the prior art wherein the fibrils arerelatively straight.

FIG. 2C describes a schematic representation of a node and fibrilmicrostructure of the prior art wherein the fibrils are bent.

FIG. 2D is a scanning electron photomicrograph (500× magnification) of anode and fibril microstructure of the prior art wherein the fibrils arebent.

FIG. 3A describes a schematic longitudinal cross sectional view of thewall of a device of the present invention having two layers of porousmaterial having bent fibrils, wherein the two layers are joined by anelastomeric adhesive.

FIG. 3B describes a transverse cross sectional view of a tubular devicemade according to FIG. 3A.

FIG. 3C is a perspective view of a tubular device shown with a dialysisneedle.

FIG. 3D is a perspective view of a sheet graft.

FIG. 4A describes a schematic longitudinal cross sectional view of thewall of a device of the present invention having two layers of porousmaterial having bent fibrils, wherein the two layers are joined by anelastomeric adhesive and wherein the elastomeric adhesive separates thetwo layers of porous material.

FIG. 4B describes a transverse cross sectional view of a tubular devicemade according to FIG. 4A.

FIG. 5A describes a schematic longitudinal cross sectional view of thewall of a device of the present invention having two layers of porousmaterial having bent fibrils, wherein the two layers are separated by alayer of an elastomeric material and wherein all layers are joined byelastomeric adhesive.

FIG. 5B describes a transverse cross sectional view of a tubular devicemade according to FIG. 5A.

FIG. 5C is a schematic longitudinal cross sectional view of the wall ofa device made as shown in FIG. 5A, differing only in having additionallayers of porous material and elastomeric material.

FIGS. 6A and 6B are analogous to FIGS. 5A and 5B respectively, with thedifference that there is little or no interpenetrating elastomericadhesive; the tubes fit together by diametrical interference, or by theuse of a non-interpenetrating adhesive, or by thermal bonding.

FIGS. 7A and 7B are scanning electron photomicrographs (50×) showing alongitudinal cross section of a cannulation site of a commerciallyavailable tubular graft of the prior art.

FIGS. 8, 9 and 10 are scanning electron photomicrographs (50×) oflongitudinal cross sections of cannulation sites of differentembodiments of tubular grafts of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view of a typical vascular graft after implantation in ahuman forearm for use as a dialysis graft 1. The relatively small radiusbend of graft 1 at its distal end is apparent.

FIG. 2A is a schematic representation of a cross section of a porousmaterial having a microstructure of nodes 2 interconnected by finefibrils 4, with void spaces between adjacent fibrils. Thismicrostructure as shown is generally typical of ePTFE. Theinterconnecting fibrils are relatively straight (the straightness isexaggerated in FIG. 2A). FIG. 2B is a scanning electron photomicrograph(500× magnification) of a node and fibril microstructure of the priorart wherein the fibrils are relatively straight.

FIG. 2C describes a schematic representation of a cross section of aporous material having a microstructure of nodes 2 interconnected byfine bent fibrils 14, again with void spaces between adjacent fibrils.FIG. 2D is a scanning electron photomicrograph (500× magnification) of anode and fibril microstructure of the prior art wherein the fibrils arebent. These materials are generally made by using materials as shown inFIGS. 2A and 2B as precursors. The precursor materials are compressedlengthwise (e.g., to about half of the length of the precursor) to causethe fibrils to bend, and then heat-treated (e.g., for 3 minutes in anoven set at 380° C.). Tubular precursors are longitudinally compressedby placing them onto a mandrel (preferably of stainless steel, Inconel®,or other heat-resistant material) over which they are a snug fit priorto subjecting the tube to longitudinal compression and heat treating;the mandrel is removed after being allowed to cool. These materialsexhibit longitudinal stretch resulting from the bent fibrils 14. Tubesmade by this method have good bending and handling properties and goodkink resistance. For longitudinally extruded and expanded tubular forms,FIGS. 2C and 2D would typically describe longitudinal cross sectionalviews.

Mean fibril lengths of these materials can be varied by knownmanufacturing methods, and may range, for example, from a few microns orless, to one hundred microns or more. The mean fibril length isdetermined from a photomicrograph of preferably a longitudinal crosssection of the sample wall, or alternatively, of a representativesurface of the sample. The mean fibril length is considered to be theaverage of ten measurements, made in the predominant direction of thefibrils of the distance between nodes connected by fibrils. The tenmeasurements are made by first verifying that the photomicrograph of arepresentative region of the sample is of adequate magnification to showat least five sequential fibrils within the length of thephotomicrograph. A series of five measurements are taken along astraight line drawn across the surface of the photomicrograph in thepredominant direction of the fibrils followed by a second series of fivemeasurements made along a second line drawn parallel to the first. Eachmeasurement constitutes the distance between adjacent nodes connected byat least one fibril. The ten measurements obtained by this method areaveraged to obtain the mean fibril length of the region.

Samples having bent fibrils should be moderately tensioned as necessaryto substantially straighten the fibrils prior to mean fibril lengthdetermination. For very thin ePTFE materials such as thin films, meanfibril lengths may be estimated by visual examination of scanningelectron photomicrographs of adequate magnification to show numerousfull-length fibrils within the boundary of the photomicrograph.

The bent character of the fibrils can also be quantified, also using aphotomicrograph as described above that is of adequate magnification toshow at least five sequential fibrils along the length of thephotomicrograph. The material sample must be in its relaxed state (i.e.,under no tension or compression) when photographed; it should be allowedto relax for 24 hours in the relaxed state at room temperature prior tobeing photographed. The photomicrograph is marked with two paralleldrawn lines spaced 24 mm apart, approximately centered on the photographand oriented so that the lines are substantially parallel to thedirection of the fibrils. Moving from left to right along the top drawnline, internodal distance “H” is determined to be the distance betweenthe node attachment points of the first distinct fibril closest to thedrawn line. A distinct fibril is one whose complete length can bevisually distinguished. Vertical displacement, a distance “V”, is nextmeasured as the perpendicular length from distance “H” to the farthestpoint on the fibril. If the fibril crossed distance “H” one or moretimes, then distance “V” is determined to be the sum of the maximumperpendicular “V” measurements. The ratio of V/H is calculated for thefibril. Moving to the right along the drawn line, “V” and “H”measurements are determined for four additional fibrils. The photographis rotated 180 degrees and the process is repeated for five additionalfibrils. Mean values of “V”, “H”, and V/H are calculated for all tenfibrils examined. Samples with bent fibrils will typically have a V/Hratio of greater than about 0.15.

FIG. 3A describes a schematic representation of a cross section of thepresent invention 30. Two ePTFE layers 32 and 34, both having bentfibrils 14, are joined by an elastomeric coating shown as dot-shadedregion 35 extending into the void space of a portion of the thickness ofboth layers adjacent to the contacting surfaces of the two ePTFE layers32 and 34. For the preferred tubular form (shown in transverse crosssection by FIG. 3B), FIG. 3A can be considered to schematicallyrepresent a longitudinal cross sectional view wherein layer 32 is anouter layer and layer 34 is an inner layer. FIG. 3C illustrates aperspective view of a tubular embodiment (showing optional middle layers38 or 39, subsequently described in detail), as it would appear about tobe cannulated by a typical dialysis needle 31. Planar or sheetembodiments of the graft 30, such as shown by the perspective view ofFIG. 3D, may be made by simply cutting finished tubes longitudinally, orby being fabricated in sheet form initially. Such sheets are relativelyflexible and can be curved appropriately to conform to the shapes ofvarious body components.

A preferred method of making the tubular embodiment begins with fittingan ePTFE precursor tube over a mandrel (preferably stainless steel ornickel-chromium-iron alloy such as Inconel®) with a slight interferencefit between the outer diameter of the mandrel and the inner diameter ofthe ePTFE tube. The tube is fitted without longitudinal compression,that is, in a longitudinally extended state with the fibrils in theirsubstantially straight conventional condition (according to FIG. 1).While ePTFE precursor tubes having bent fibrils may be used, they arenot required. A relatively thin coating of a desired elastomericmaterial is then applied to the outer surface of the ePTFE tube fittedover the mandrel. A preferred elastomeric material is MED-1137 AdhesiveSilicone Type A from NuSil Silicone Technology (Carpenteria, Calif.).The application of the elastomeric adhesive may be accomplished byvarious means such as spraying, dip coating, brushing or by spreadingwith gloved fingers. After the outer surface of the inner ePTFE tube hasbeen coated with the elastomeric adhesive, a second ePTFE tube is fittedover the first, preferably with a small amount of interference betweenthe inside diameter of the outer ePTFE tube and the outer diameter ofthe inner tube. The pair of coaxially-fitted ePTFE tubes are thenlongitudinally compressed while still fitted over the mandrel. Theamount of longitudinal compression is a function of the desired amountof longitudinal stretch in the completed graft; more longitudinalcompression provides a greater amount of longitudinal stretch. Adesirable amount of longitudinal compression may be, for example, about100 percent (i.e., the coaxial tubes are compressed to half of theiroriginal length). The amount of longitudinal compression may thus be10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, 100%, 110%, 120%, 130%,140%, 150%, 160%, 170%, 180%, 190%, 200%, etc. (i.e., until wrinklingand significant non-uniform deformation of the tube occurs).

Following longitudinal compression, the adhesive is cured. It may beallowed to cure at ambient temperature or may be cured by other meanssuch as the use of heat above ambient.

Following curing, the coaxial tubes 32 and 34 are removed from themandrel. The adhesive will have interpenetrated the fitted surfaces ofthe tubes to some portion of the thickness of each tube. This results ingood mechanical adhesion of the coaxial tubes. The presence of the curedelastomeric material holds the coaxial tubes 32 and 34 in a state oflongitudinal compression whereby the length of the resulting coaxialgraft 30 is less than that of the longitudinally extended length of theprecursor tubes as fitted over the mandrel prior to the longitudinalcompression step. Both the inner tube 34 and outer tube 32 have bentfibrils 14 resulting from the cured elastomeric material holding thecoaxial tubes in a longitudinally compressed state.

The bent fibrils 14 provide the resulting graft 30 with good handlingand bending properties, as well as kink resistance. The combination ofthe bent fibrils 14 of the two ePTFE layers 32 and 34 and the presenceof the silicone adhesive at the joined surfaces of the two ePTFE tubes(region 35 of FIG. 3A) provides reduced bleeding at a needle puncturesite during use.

The chosen tubes may have fibril lengths as desired; it is not requiredthat both tubes have equivalent fibril lengths. Tubes having differentfibril lengths at their inner and out surfaces are known and may be usedfor either or both tubes.

The selected tubes may have annular or helically-oriented densifiedsegments alternating with un-densified segments along their length foradditional hoop strength if desired. A preferred process for making sucha radially supported ePTFE tubular structure is sequentially describedas follows. A longitudinally extruded and expanded ePTFE tube isobtained and fitted coaxially over a mandrel having an outside diameterthe same as or slightly larger than the inside diameter of the ePTFEtube. The ends of the tube are then pushed together so that the lengthof the tube is at least about 50%, and preferably about 20%, of theoriginal length of the tube prior to this longitudinal compression. Thetube and mandrel are then heated in an air convection oven set at 380°C. for approximately 50 seconds. Next, predetermined regions of thecompressed tube are heat-treated via the use of a laser (e.g., a model2010, 20W CO₂ laser with a 6.35 mm focal length lens, Applied LaserTechnology, Inc., Scottsdale, Ariz.) directed toward the rotatingsurface of the tube where a densified region is desired. Subsequent tothe laser treatment and cooling, the graft is removed from the mandrel.With moderate tension applied to the ends of the graft, the portions nottreated by the laser readily extend out to their original length. Theportions treated by the laser, however, are not readily extendible.These denser portions provide the radial support to the graft.

Optionally, either or both tubes may have a helical wrap of ePTFE filmfor increased hoop strength. The outer surface of the coaxial graft mayalso be provided with a reinforcing structure such as rings or a helicalstructure of a material such as non-porous PTFE or fluorinated ethylenepropylene (FEP). The reinforcing rings (or the reinforcing helicalstructure) may optionally be provided so as to be removable duringsurgery. The reinforcing material may be metal, such as nitinol wire(e.g., as a braided structure with fine wire) or stainless steel, or maybe plastic or other suitable material. It is apparent that a reinforcingstructure may be provided within the coaxial structure, between thejoined surfaces of the inner and outer tubes.

It is also apparent that the tubular structure may be used as the graftcomponent of a stent graft when fitted to a stent component. The stentmay be exterior to the graft, or the graft may be exterior to the stent.Likewise, the stent component may be provided between the two coaxialtubes, in the region of the interface.

The graft may be provided with a variety of therapeutic agents for avariety of purposes, such as anti-inflammatory, anti-bacterial oranti-thrombogenic drugs. Such agents and treatments are known in thevascular graft and stent fields.

The chosen tubes may have wall thicknesses as desired. Their wallthicknesses may be the same or they may be chosen to be different.Generally, it is preferred that the combined wall thicknesses berelatively thin for good handling, for example, about 0.8 mm or less incombined thickness (e.g., a wall thickness of about 0.7 mm, 0.6 mm, 0.5mm, 0.4 mm, 0.3 mm, 0.2 mm or 0.1 mm). It is apparent that a greaterwall thickness generally improves the reduction in puncture sitebleeding, but at the expense of the surgical handling properties of thegraft. Therefore, the chosen total wall thickness may be something of acompromise between those characteristics.

The amount of interpenetration of the elastomeric adhesive into thejoined surfaces of the coaxially-fitted porous tubes is a function ofvariables including the amount of elastomeric material applied, themethod of application, the type of elastomeric material chosen, curingtechnique and the viscosity of the elastomeric material. It is apparentthat these variables will also affect the graft handling and the amountof reduction in puncture site leakage that will be achieved. It islikewise apparent that the degree of interpenetration may be keptminimal in order that only a small percentage of the thickness of eachof the inner and outer tubes is affected. Alternatively, the adhesivemay interpenetrate to the outer and/or inner surfaces of the coaxialgraft. The adhesive may be provided so as to maintain the porouscharacter of the graft through its thickness (i.e., between the innerand outer surfaces). Alternatively, the adhesive may be applied torender the coaxial graft impervious through its thickness, precludingthe passage of body fluids or other biologic components through thethickness of the graft.

As shown by the schematic longitudinal cross section of FIG. 4A and thetransverse cross section of the tubular embodiment of FIG. 4B, theadhesive may be applied in an amount that results in a layer 38 ofadhesive between inner layer 34 and outer layer 32, so that the innersurface of outer layer 32 is separated from and not in contact with theouter surface of inner layer 34. The adhesive may still interpenetrateinto the void spaces of both layers 32 and 34 beyond these surfaces, asindicated by dot-shaded adhesive-coated region 36.

FIG. 5A describes a schematic representation of a cross section of analternative embodiment wherein graft 30 includes a discrete layer 39 ofelastomeric material. The preferred embodiment is again tubular (shownby the transverse cross section of FIG. 5B), for which FIG. 5A wouldrepresent a longitudinal cross section of the wall of such a tubulargraft 30. Discrete layer 39 of elastomeric material is preferablyadhesively joined to the inner surface of outer tube 32 and the outersurface of inner tube 34 by an elastomeric adhesive.

The tubular embodiment of FIGS. 5A and 5B is made by fitting a selectedePTFE tube over a mandrel with a slight interference fit. The ePTFE tubeis fitted in its extended state (without longitudinal compression) sothat the fibrils are in a substantially straight condition. The outersurface of the ePTFE tube is coated with elastomeric adhesive asdescribed above. A length of elastomeric tubing (e.g., silicone tubing)is obtained having an inside diameter of about equal dimension to theoutside diameter of the ePTFE tube fitted over the mandrel. The lengthof elastomeric tubing should be greater than the length of the mandrel.The length of elastomeric tubing is carefully fitted over theadhesive-coated outer surface of the ePTFE tube. The ends of theelastomeric tubing should extend beyond the ends of the mandrel. Anoverhand knot is tied in one end of the elastomeric tubing that extendsbeyond the end of the mandrel. Tension is applied to the length ofelastomeric tubing from the end opposite the knot. The tension ismaintained by securing the tensioned end to the same end of the mandrelby suitable means. For example, while maintaining tension so that theelastomeric tubing is in a longitudinally-stretched state, anotheroverhand knot is tied in the opposite end of the elastomeric tubing thatextends beyond the end of the mandrel opposite the first knot, therebymaintaining the tension on the elastomeric tubing by the pair of knotssecuring the elastomeric tubing beyond the ends of the mandrel.

The outer surface of the tensioned elastomeric tubing is then coatedwith elastomeric adhesive, after which a second ePTFE tube is fittedunder a small amount of tension over the tensioned elastomeric tubing.Both coatings of the elastomeric adhesive are then cured (or allowed tocure) while the intermediate layer of elastomeric tubing remains intension. After curing is complete, one of the knots in the elastomerictubing may be untied from one end of the mandrel or alternatively may betransversely cut free of the remainder of the coaxial construct,allowing the construct to be removed from the opposite end of themandrel. Following removal, both ends of the construct may be cuttransversely to dispose of the second knot and provide a graft withsquarely-cut clean ends.

When the first knot in the elastomeric tubing is removed by untying orcutting, the tension in the length of the elastomeric tubing is releasedand the elastomeric tubing will recover most or all of its original,shorter pre-tension length. This recovery applies compression to theinner 34 and outer 32 ePTFE tubes, causing the fibrils of the ePTFEtubes 32 and 34 to become bent fibrils 14.

It is apparent that the amount of longitudinal stretch available in thecompleted graft 30 will be a function of the amount of tension orstretch applied to the elastomeric tube during the construction process.This amount may be described as a function of the length change betweenthe transversely cut ends of the graft as measured prior to cutting andreleasing the tension on the stretched elastomeric tube, and measuredagain after cutting and removal of the completed graft from the mandrel.

The bent fibrils 14 of the resulting graft 30 also provide thisembodiment with good handling and bending properties, as well as kinkresistance. The combination of the bent fibrils 14 of the two ePTFElayers 32 and 34 and the presence of the elastomeric adhesive at thejoined surfaces of the two ePTFE tubes and the discrete layer 39 of theelastomeric tubing provides reduced bleeding at a needle puncture siteduring use.

While the preferred embodiment of the graft is made with two layers ofgraft material and an intermediate elastomeric layer, the graft may alsohave additional layers. For example, three layers of graft material maybe used alternating with two layers of elastomeric material. FIG. 5C isa schematic longitudinal cross sectional view of the wall of a graftmade as shown in FIG. 5A, differing only in having additional layers ofporous material and elastomeric material. It is apparent that variouscombinations of multiple layers may be created as desired for any of thevarious embodiments.

For all embodiments, the graft layers may all be of the same material orthe graft layers may be different in one or more characteristics (e.g.,wall thickness or mean fibril length). Likewise, if more than a singleelastomeric layer is used, the layers may be the same or may havedifferent characteristics.

FIGS. 6A and 6B describes a schematic cross sectional representation ofan alternative to that of FIG. 4A wherein no interpenetratingelastomeric adhesive is used and the ePTFE tubes are simply aninterference fit with the elastomeric tubular layer 39. Likewise, anadhesive may be used in place of the interference fit wherein theadhesive is applied as a discontinuous pattern such as a dot matrix, andthus only interpenetrates the area that it contacts the ePTFE tubesurfaces. Such a discontinuous adhesive would not be required to be anelastomeric adhesive.

Various examples of the present invention were manufactured usingtubular graft materials of different types (e.g., wall thickness) andwith different elastomeric materials for the intermediate layer.Following manufacture (including curing of the elastomeric materials),the completed grafts were subjected to a cannulation test wherein thegraft was subjected to water pressure and cannulated with a needle. Theresults of such a test are considered as comparative indicators only,due to the entirely different behavior of such cannulated grafts whenimplanted and containing flowing blood under pressure.

Testing generally consisted of pressurizing each example individuallywith water at 150 mm Hg at room temperature, cannulating the sampledevice with a 15 gauge dialysis needle, removing the needle and applyingdigital pressure to the needle hole for a short period, typically about10 seconds. After removal of the digital pressure, the flow rate(ml/minute) of water escaping from the needle hole was measured using apositive displacement flow sensor (Cole-Parmer Instruments Model No.MA0-125-T-20-AA connected to a digital display). Due to thenon-linearity of the flow meter, the calibrated system is used withappropriate correction factors applied as required.

A vertical water column was used to supply the pressure for this leaktesting of each example. Each example was tested in a horizontalposition and at the same elevation with respect to the water column. Theexample graft to be tested was connected to the base of the water columnby a short length of tubing having a barbed fitting at the end of thetubing to which the graft was fitted by interference. The opposite endof the example graft was clamped closed using a forceps. Each testedexample was checked to ensure that it did not leak any water prior tocannulation. A randomly selected location between the graft ends wascannulated with a new 15 gauge stainless steel needle (Monoject 200aluminum hub hypodermic 15×1.5 inch B bevel, Sherwood Medical, St.Louis, Mo.). The needle was then removed and digital pressure wasapplied to the location of the needle hole for about ten seconds andreleased for 40 seconds. This use of digital pressure was intended tosimulate any effect resulting from the conventional use of digitalpressure on dialysis patients. Digital pressure was again applied forabout one second, released for one second, applied again for one secondand released. Seventy seconds after final release of digital pressure,the indicated flow rate was recorded.

Cannulation was accomplished by inserting the point of the needle intoan upper surface of the graft with the bevel of the needle facingupwards, that is, away from the surface of the graft. The point of theneedle was inserted through the graft so as to intersect thelongitudinal axis of the graft. The needle was always aligned with thegraft so that the longitudinal axis of the needle and the longitudinalaxis of the vascular graft lay in a common plane during cannulation.Each needle was oriented at an angle of about 45 degrees with respect tothe longitudinal axis of the graft. Care was taken not to damage theopposite or lower surface of any tested example during cannulation ofthe upper surface.

Conventional 6mm inside diameter ePTFE vascular grafts will show a leakflow rate of typically greater than about 200 ml/minute when cannulatedon this test fixture by the described method.

EXAMPLE 1

An example was made generally according to FIGS. 5A and 5B. Two 15 cmlengths of 6 mm inside diameter ePTFE tubing, made by longitudinalextrusion and expansion (i.e., with fibrils that are substantiallyparallel to each other and oriented in a direction parallel to thelength of the tubing), were cut from a longer length. This tubing had awall thickness of about 0.4 mm and a mean fibril length of about 22microns.

One of the 15 cm lengths of ePTFE tubing was fitted over a stainlesssteel mandrel that provided a slight interference fit. Tension wasapplied to the ends of the ePTFE tube by hand to assure that the tubewas not under any longitudinal compression and that the fibrils of themicrostructure were substantially straight. This tensioning step was notdeemed to be critical but helped ensure uniformity of the resultinggraft.

Both ends of the ePTFE tube were temporarily secured to the mandrel byhelically wrapping a strip of thin ePTFE film around each end of thetube. A coating of medical grade silicone adhesive (Adhesive SiliconeType A, Med-1137, NuSil Silicone Technology, Carpenteria Calif., diluted20% by volume with heptane) was applied to the outer surface of theePTFE tube using gloved human fingers.

A 30 cm length of silicone tubing was obtained (Part No.T050PLAT374×354, Jamak Corp., Weatherford, Tex.). This tubing had aninside diameter of about 9.0 mm and an outside diameter of about 9.5 mm.This length of silicone tubing was carefully fitted over theadhesive-coated outer surface of the ePTFE tube. An overhand knot wastied in one end of the silicone tube that protruded beyond the end ofthe mandrel. Tension was applied to the opposite end of the siliconetube that protruded beyond the opposite end of the mandrel, causing thesilicone tubing to neck down with regard to its inside diameter and comeinto full contact with the underlying adhesive-coated surface of theePTFE tube. This opposite end of the tensioned silicone tube was thentemporarily secured to the outer surface of the mandrel (extendingbeyond the end of the ePTFE tube) by a tightly-wound wrapping of a stripof thin ePTFE film, applied by hand. A coating of the same siliconeadhesive was then applied to the outer surface of the tensioned siliconetubing.

The second 15 cm length of ePTFE tubing was forced onto the outersurface of an 8 mm diameter stainless steel mandrel, thereby distendingthe 6mm inside diameter ePTFE tube diametrically. The diametricallydistended 15 mm length of ePTFE tube was then carefully fitted over theadhesive-coated, tensioned silicone tubing working from the end securedto the mandrel by the ePTFE film wrapping. When coaxial with the innerePTFE tube, slight tension was applied to the ends of the outer ePTFEtube to assure that there was no longitudinal compression on the ePTFEtube and that the fibrils of the microstructure were substantiallystraight. Finally, an outer helical wrap of ePTFE film was appliedtemporarily to the outer surface of the outer ePTFE tube to ensure goodcontact between the three underlying layers.

The entire construction process was completed relatively quickly, beforeany significant curing of the silicone adhesive layers was effected. Oncompletion of this construction, it was set aside overnight, in a roomtemperature environment, to allow the two layers of silicone adhesion tocure.

After adhesive curing, the outer helical wrap of ePTFE was removed. Bothends of the resulting graft were trimmed by cutting transversely throughall three layers adjacent the ends of the ePTFE tubes with a sharpblade, after which the graft was removed from the mandrel. The length ofthe resulting graft was about 55% of the original length of the ePTFEtubes due to axial compression applied to those tubes by the releasingof the previously-applied axial tension on the silicone tubing. Theresulting graft had a wall thickness of about 1.0 mm as measured by theopposing flat faces of the jaws of calibrated digital calipers (used forall example wall thickness measurements described herein). Whensubjected to the dialysis needle cannulation test, the graftdemonstrated a leakage of about 16 ml/minute. The graft showed goodbending properties and offered longitudinal stretch.

EXAMPLE 2

Another example was created in the same manner as described for Example1 except that both ePTFE tubes were tubes of greater wall thickness andhad been processed by being longitudinally compressed and heat-treatedto provide them with bent fibrils prior to construction of the example.These precursor tubes had a wall thickness of about 0.6 mm.

The resulting graft had a wall thickness of about 1.0 mm. It appearedthat the wall thickness was the result of the relatively tight overwrapof the temporarily applied helical film wrap used during construction.When subjected to the dialysis needle cannulation test, the graftdemonstrated a leakage of about 19 ml/minute. The graft showed goodbending properties and offered longitudinal stretch.

EXAMPLE 3

An example of a tubular graft generally according to FIGS. 3A and 3B wasmade using the same ePTFE tubing as used for Example 1. A length of thisePTFE tubing was fitted over a mandrel and secured as described forExample 1, and coated with the same silicone adhesive in the samemanner. The second tube was, also as described for Example 1, distendedto an inside diameter of 8 mm. The second ePTFE tube was then coaxiallyfitted over the first, adhesive-coated ePTFE tube, with tension appliedto the ends of the second tube to assure that it was fully extendedlongitudinally (with the fibrils of the microstructure in asubstantially straight condition). A temporary helical wrap of a stripof thin ePTFE film was uniformly applied about the outer surface of theouter tube. The coaxial tubes were then longitudinally compressedtogether, by applying a compressive force to both tubes by pushing theopposing ends of the tubes toward each other by hand. No wrinkling orgross deformation of the tubes resulted, in part due to the temporaryhelical wrap of ePTFE film. The compressed length of the coaxial tubeswas slightly less than half of the length prior to longitudinalcompression. The resulting assembly was set aside overnight at roomtemperature to allow the silicone adhesive to cure.

Following overnight curing, the temporary ePTFE helical film wrap wasremoved. The ends of the graft were trimmed by cutting transversely witha sharp blade, after which the graft was removed from the mandrel.

The resulting graft had a wall thickness of about 0.9 mm. When subjectedto the dialysis needle cannulation test, the graft demonstrated aleakage of about 50 ml/minute. The graft showed reasonably good bendingproperties, and offered longitudinal stretch.

When two samples made according to this description were stretched usingan Instron® tensile tester at a rate of 100 mm/min, the samplesstretched an average of about 12.5% under a 0.25 kg force, about 36.5%under a 0.5 kg force, and about 54% under a 1 kg force (Instron® ModelNo. 5564 (Canton, Me.) fitted with a 10N load cell and Part No. 2712-002pneumatically-operated grips (pressure supplied at 276 KPa) with PartNo. 2702-003 knurled 25 mm by 12 mm faces, with the long axis of theface oriented to be parallel to the test axis). Both grafts quicklyrecovered to about their original length on the release of the lengthextending force.

Two additional samples were made according to this description, with oneof the two samples longitudinally compressed as described for thisexample and the other not compressed. The result was that the first ofthese two grafts had bent fibrils and the second did not. Their bendingbehavior was compared by gently and progressively bending each sample incomparison to plastic templates with semi-circular cut-outs of differentradii, in increments of 1.6 mm ( 1/16 inch), noting the radius at whicheach sample kinked. Each of these gauges thus defined the outside radiusof the bend, that is, the radius of the outer meridian of the bent tube.The tubular sample with the bent fibrils kinked at a bend radius ofabout 14.3 mm, while the sample with the straight fibrils kinked at abend radius of about 31.8 mm. The tubular sample of the presentinvention could thus be bent at radii of 30 mm, 25 mm, 22 mm, 20 mm, 19mm, 18 mm, 17 mm, 16 mm and 15 mm without kinking. The advantage of thebent fibrils in the composite construction was apparent. It isanticipated that further refinement of bending properties is possible.

EXAMPLE 4

An example was made similarly to Example 1, except that the outer ePTFEtube was replaced with a thinner ePTFE film tube made by helicallywrapping two layers of a strip of ePTFE film (about 2.5 cm width, 50micron fibril length, 0.01 mm thickness and 0.3 g/cc density) about thesurface of a 10 mm diameter mandrel. The film was applied in a“bias-ply” fashion by helically wrapping first in one direction, thenreturning, back over the first wrap. The pitch of the helical wrapresulted in adjacent edges of the helical wrap being about 2.5 mm apartas measured in the direction of the tube length. The film-wrappedmandrel was heated for 10 minutes in an air convention oven set at 370°C., removed from the oven and allowed to cool. After cooling, thehelically-wrapped film tube was removed from the mandrel.

The film tube was carefully fitted over the outer surface of theadhesive-coated, tensioned silicone tube. Tension was applied to theends of the film tube, causing it to neck down in diameter and conformto the outer surface of the underlying adhesive-coated silicone tube.The outer surface of this assembly was then temporarily helicallywrapped with another layer of ePTFE film and set aside overnight tocure. Following curing, the temporary outer film wrap was removed. Theends of the graft were trimmed transversely as described above and thegraft removed from the mandrel.

The resulting graft had a wall thickness of about 0.8 mm. When subjectedto the dialysis needle cannulation test, the graft demonstrated aleakage of about 50 ml/minute. The graft showed good bending propertiesand offered longitudinal stretch.

EXAMPLE 5

An example was made as described for Example 1 with the difference thatboth ePTFE tubes were thinner, having a wall thickness of about 0.1 mm.At the end of the construction process, while the assembled graft wasstill on the mandrel, the outer surface was provided with indicia alongits length marking length intervals of 1.0 cm. Following curing of theelastomeric adhesive and removal of the finished graft from the mandreland transverse trimming of the graft ends, the distance between theindicia was again measured. This distance, without any tension on thegraft (i.e., in a relaxed state), was about 5.5 mm.

The resulting graft had a wall thickness of about 0.6 mm. When subjectedto the dialysis needle cannulation test, the graft demonstrated aleakage of about 10 ml/minute. The graft showed good bending propertiesand offered longitudinal stretch.

EXAMPLE 6

An example was made as described for Example 4 with the difference thatthe inner ePTFE tube was thinner, having a wall thickness of about 0.1mm. At the end of the construction process, while the assembled graftwas still on the mandrel, the outer surface was provided with indiciaalong its length marking length intervals of 10 mm. Following curing ofthe elastomeric adhesive and removal of the finished graft from themandrel and transverse trimming of the graft ends, the distance betweenthe indicia was again measured. This distance, without any tension onthe graft (i.e., in a relaxed state), was about 5.5 mm.

The resulting graft had a wall thickness of about 0.4 mm. When subjectedto the dialysis needle cannulation test, the graft demonstrated aleakage of about 28 ml/minute. The graft showed good bending propertiesand offered longitudinal stretch.

EXAMPLE 7

An example was made generally as described by Example 3 except that,prior to fitting the silicone tube over the adhesive-coated inner ePTFEtube, a braided nitinol wire tube was fitted over the inner ePTFE tube.No outer ePTFE tube was provided, leaving the silicone tube as the outertube. The braided wire tube was made of 0.1 mm diameter nitinol wire(Part No. SE 508, NDC, Inc., Fremont, Calif.). Braiding was accomplishedon a conventional machine for making tubular braids (Steeger USA, Inc.,Spartanburg, S.C.), using 32 individual strands of this wire with abraid density of about 16 picks per cm.

It was apparent that this sample could have optionally been providedwith an outer ePTFE tubular covering.

The resulting graft had a wall thickness of about 0.8 mm. When subjectedto the dialysis needle cannulation test, the graft demonstrated zeroleakage. The graft showed good bending properties and offeredlongitudinal stretch.

FIGS. 7A-10 are scanning electron photomicrographs (50×) of longitudinalcross sections of cannulation sites of various tubular vascular grafts.All of the grafts shown in these photomicrographs were cannulated by thesame person while the grafts were pressurized with water at roomtemperature as described above for testing of the various examples. Thedialysis needles were of 1.5 mm diameter and were of the same type asdescribed above for testing of the examples. Dialysis needles were usedfor a maximum of two punctures and then discarded. Cannulation wasperformed by the same technique described above for the examples.

FIGS. 7A and 7B are scanning electron photomicrographs of a longitudinalcross section through the same cannulation site of a commerciallyavailable graft of the prior art, a 6 mm GORE-TEX® Stretch Thin WallVascular Graft. While the cannulation of this sample resulted in arelatively small remaining hole through the graft wall following removalof the dialysis needle, as seen in the photomicrograph, the width of thehole was still significant (appearing to be greater than about 200microns, in comparison to the 1500 micron needle diameter). FIGS. 8, 9and 10 are photomicrographs of longitudinal cross sections ofcannulation sites of embodiments of grafts of the present inventionmade, respectively, as described above by Examples 1, 3 and 5. It isseen that, following removal of the dialysis needle, the resultingaperture appears closed. As described previously, all of these graftsdisplayed low leakage when tested while simultaneously offering goodhandling properties to a surgeon.

While the principles of the invention have been made clear in theillustrative embodiments set forth herein, it will be evident to thoseskilled in the art to make various modifications to the structure,arrangement, proportion, elements, materials and components used in thepractice of the invention. To the extent that these variousmodifications do not depart from the spirit and scope of the appendedclaims, they are intended to be encompassed therein.

1. An implantable device comprising: a.) an inner layer of porousmaterial having a microstructure of nodes and fibrils wherein asubstantial portion of said fibrils are in a bent configuration; b.) amiddle layer of an elastomeric material; and c.) an outer layer ofporous material having a microstructure of nodes and fibrils wherein asubstantial portion of said fibrils are in a bent configuration.
 2. Animplantable device according to claim 1 wherein said device is tubular.3. An implantable device according to claim 1 wherein at least one ofsaid inner and outer layers comprise expanded polytetrafluoroethylene.4. An implantable device according to claim 3 wherein said device istubular.
 5. An implantable device according to claim 4 wherein at leastone of said inner and outer layers is provided with a helical wrap ofexpanded polytetrafluoroethylene film about at least a portion of anouter surface of said layer.
 6. An implantable device according to claim4 wherein said elastomeric material comprises silicone.
 7. Animplantable device according to claim 4 wherein said elastomericmaterial comprises polyurethane.
 8. An implantable device according toclaim 4 wherein said elastomeric material comprises a fluoroelastomer.9. An implantable device according to claim 4 wherein said devicefurther comprises nitinol.
 10. An implantable device according to claim4 wherein said inner and outer layers of porous material each have athickness and said elastomer extends into void spaces of the porousmaterial for a portion of said thickness.
 11. An implantable deviceaccording to claim 4 wherein said inner and outer layers each has alength and at least one of said inner and outer layers has regions ofhigher density alternating with regions of lower density alternatingalong said length.
 12. An implantable device according to claim 4wherein at least one of said inner and outer layers comprises expandedpolytetrafluoroethylene film.
 13. An implantable device according toclaim 1 wherein said elastomeric material comprises silicone.
 14. Animplantable device according to claim 1 wherein said elastomericmaterial comprises polyurethane.
 15. An implantable device according toclaim 1 wherein said elastomeric material comprises a fluoroelastomer.16. An implantable device according to claim 1 wherein said devicefurther comprises nitinol.
 17. An implantable device according to claim1 wherein said inner and outer layers of porous material each has athickness and said elastomer extends into void spaces of the porousmaterial for a portion of said thickness.
 18. An implantable deviceaccording to claim 1 wherein said inner and outer layers each has alength and at least one of said inner and outer layers has regions ofhigher density alternating with regions of lower density along saidlength.
 19. A tubular implantable device comprising first and secondtubes having inner and outer surfaces and a microstructure includingfibrils between the inner and outer surfaces, wherein said second tubeis arranged concentrically around said first tube, wherein the outersurface of said first tube and the inner surface of said second tube areadhered with an elastomeric material, and wherein at least a substantialportion of the fibrils of the first and second tubes are in a bentconfiguration.
 20. A tubular implantable device according to claim 19wherein at least one of said first and second tubes comprises expandedpolytetrafluoroethylene.
 21. A tubular implantable device according toclaim 20 wherein at least one of said first and second tubes is providedwith a helical wrap of expanded polytetrafluoroethylene film about atleast a portion of the outer surface of said tube.
 22. A tubularimplantable device according to claim 20 wherein said elastomericmaterial comprises silicone.
 23. A tubular implantable device accordingto claim 20 wherein said elastomeric material comprises polyurethane.24. A tubular implantable device according to claim 20 wherein saidelastomeric material comprises a fluoroelastomer.
 25. A tubularimplantable device according to claim 20 wherein said first and secondtubes each has a thickness and said elastomer extends into void spacesof the microstructure for a portion of said thickness.
 26. A tubularimplantable device according to claim 20 wherein said first and secondtubes each has a length and at least one of said first and second tubeshas regions of higher density alternating with regions of lower densityalong said length.
 27. A tubular implantable device according to claim20 wherein at least one of said inner and outer layers comprisesexpanded polytetrafluoroethylene film.
 28. A tubular implantable deviceaccording to claim 19 wherein said elastomeric material comprisessilicone.
 29. A tubular implantable device according to claim 19 whereinsaid elastomeric material comprises polyurethane.
 30. A tubularimplantable device according to claim 19 wherein said elastomericmaterial comprises a fluoroelastomer.
 31. A tubular implantable deviceaccording to claim 19 wherein said first and second tubes each has athickness and said elastomer extends into void spaces of themicrostructure for a portion of said thickness.
 32. The tubularimplantable device of claim 19 wherein a tubular layer of elastomericmaterial is located between the outer surface of the first tube and theinner surface of the second tube.
 33. A tubular implantable deviceaccording to claim 32 wherein at least one of said first and secondtubes comprises expanded polytetrafluoroethylene.
 34. A tubularimplantable device according to claim 33 wherein at least one of saidfirst and second tubes is provided with a helical wrap of expandedpolytetrafluoroethylene film about at least a portion of the outersurface of said tube.
 35. A tubular implantable device according toclaim 33 wherein said tubular layer of elastomeric material comprisessilicone.
 36. A tubular implantable device according to claim 33 whereinsaid tubular layer of elastomeric material comprises polyurethane.
 37. Atubular implantable device according to claim 33 wherein said tubularlayer of elastomeric material comprises a fluoroelastomer.
 38. A tubularimplantable device according to claim 33 wherein said tubular layer ofelastomeric material comprises nitinol.
 39. A vascular graft fordialysis comprising at least two layers of expandedpolytetrafluoroethylene with an elastomeric material between the twolayers of expanded polytetrafluoroethylene, the vascular graft having abend radius of 30 mm without kinking, wherein the vascular graft isadapted for cannulation with a dialysis needle within 48 hours afterimplantation.
 40. A vascular graft according to claim 39 wherein thevascular graft is adapted for cannulation with a dialysis needle within24 hours after implantation.
 41. A vascular graft according to claim 39having a bend radius of 25 mm without kinking, wherein the vasculargraft is adapted for cannulation with a dialysis needle within 48 hoursafter implantation.
 42. A vascular graft according to claim 41 whereinthe vascular graft is adapted for cannulation with a dialysis needlewithin 24 hours after implantation.
 43. A vascular graft according toclaim 39 having a bend radius of 20 mm without kinking, wherein thevascular graft is adapted for cannulation with a dialysis needle within48 hours after implantation.
 44. A vascular graft according to claim 43wherein the vascular graft is adapted for cannulation with a dialysisneedle within 24 hours after implantation.
 45. A vascular graftaccording to claim 39 having a bend radius of 17 mm without kinking,wherein the vascular graft is adapted for cannulation with a dialysisneedle within 48 hours after implantation.
 46. A vascular graftaccording to claim 45 wherein the vascular graft is adapted forcannulation with a dialysis needle within 24 hours after implantation.47. A vascular graft according to claim 39 having a bend radius of 15 mmwithout kinking, wherein the vascular graft is adapted for cannulationwith a dialysis needle within 48 hours after implantation.
 48. Avascular graft according to claim 47 wherein the vascular graft isadapted for cannulation with a dialysis needle within 24 hours afterimplantation.
 49. A vascular graft for dialysis comprising at least twolayers of expanded polytetrafluoroethylene with an elastomeric materialbetween the two layers of expanded polytetrafluoroethylene, the expandedpolytetrafluoroethylene having a microstructure of substantially bentfibrils, wherein the vascular graft is adapted for cannulation with adialysis needle within 48 hours after implantation.
 50. A vascular graftaccording to claim 49 wherein the vascular graft is adapted forcannulation with a dialysis needle within 24 hours after implantation.51. A method of making a tubular implantable device comprising: a)providing first and second expanded polytetrafluoroethylene tubes, saidtubes having outer surfaces; b) fitting said first expandedpolytetrafluoroethylene tube over a mandrel with a slight interferencefit; c) coating at least a portion of the outer surface of said firsttube with an elastomeric material; d) fitting said second expandedpolytetrafluoroethylene tube over the coated surface of said first tube;e) longitudinally compressing said first and second expandedpolytetrafluoroethylene tubes; f) curing said elastomeric material; andg) removing said first and second tubes from said mandrel.
 52. A methodof making a tubular implantable device comprising: a) providing firstand second tubes of expanded polytetrafluoroethylene, said tubes havingouter surfaces; b) providing an elastomeric tube having an outersurface; c) fitting said first tube of expanded polytetrafluoroethyleneover a mandrel with a slight interference fit; d) coating at least aportion of the outer surface of said first tube with an elastomericmaterial; e) fitting said elastomeric tube over the first expandedpolytetrafluoroethylene tube; f) applying tension longitudinally to saidelastomeric tube and restraining both ends of said elastomeric tube tomaintain said elastomeric tube in a state of tension; g) coating atleast a portion of the outer surface of said elastomeric tube with anelastomeric material; h) fitting said second expandedpolytetrafluoroethylene tube over the coated surface of said elastomerictube; i) curing said elastomeric material; and j) removing all threetubes from said mandrel and trimming both ends of the resulting tubularimplantable device.